In a variety of industries, a variety of attempts has been made worldwide to reduce environmental impacts and burdens. In particular, in the automobile industry, development for promoting the spread of not only fuel-efficient gasoline engine vehicles, but also so-called eco-friendly vehicles, such as hybrid vehicles and electric vehicles, as well as for further improving the performance of such vehicles has been advanced day by day.
Measurement of the fuel consumption performance of vehicles is conducted by detecting the concentration of oxygen in the measurement target gas, such as the exhaust gas, using a gas sensor, and determining the difference between the concentration of oxygen and the concentration of oxygen in the air as a reference gas.
As a specific structure of an embodiment of a gas sensor element that constitutes such a gas sensor, an element that generally includes the following is typically known: a detection portion including a solid electrolyte layer having on the opposite sides thereof a pair of electrodes including an electrode on the measurement target gas side and an electrode on the reference gas side, a porous diffusive resistance layer (or a diffusion-controlled layer) that surrounds the electrode on the measurement target gas side with a measurement target gas space interposed therebetween, a shielding layer that defines the measurement target gas space with the porous diffusive resistance layer, and a reference gas space protective layer that surrounds the electrode on the reference gas side with a reference gas space interposed therebetween; a heat-generating portion including a heat generation source, such as a heater; and a porous protective layer (or a catalyst-carrying protective layer or a catalyst-carrying trapping layer) that surrounds the detection portion and the heat-generating portion. Output current is determined by controlled diffusion in which rich gas, such as oxygen or HC, for example, reaches the electrode on the measurement target gas side via the porous diffusive resistance layer. In the case of an A/F sensor, for example, the A/F value is detected.
The aforementioned gas sensor detects the concentration of oxygen in the exhaust gas under a high temperature atmosphere of greater than or equal to 700° C. Thus, if water droplets in the exhaust gas collide with the gas sensor element of the gas sensor, thermal shock may be generated due to partial quenching, and due to a change in the volume of the element with a change in the temperature, water-induced cracking of the element may occur, so that the sensing function may be lost, which is problematic. If a gas sensor element with the aforementioned configuration in which the detection portion and the like are surrounded by the porous protective layer is applied to address such a problem, it becomes possible to effectively suppress collision of water droplets with the detection portion and the heat-generating portion owing to the catalyst-carrying protective layer. It should be noted that Patent Literatures 1 and 2 each disclose a technology related to a gas sensor element in which the periphery of the element is surrounded by a porous protective layer made of alumina.
By the way, if inferior fuel that is distributed in some areas is used for gasoline for the aforementioned fuel-efficient gasoline engine vehicles or hybrid vehicles, it is concerned that the performance of the vehicles may degrade due to SOx poisoning (or S poisoning) that would occur with an increase in the S components in the fuel.
Thus, it is an urgent object to be achieved in the technical field to develop a gas sensor element that can, even when the gas sensor element is poisoned with S and thus may have a reduced sensor output, suppress a reduction in the sensor output due to the S poisoning.